Mary did not know in the least how to discover the doors.

In fact, she supposed that there were not any.

She would have to retrace the way she came--perhaps even ask the terrible Lady Drummond how to get out.
She looked up at the long windows with the eye of a bird who has strayed in and cannot find the way out.

The elusive air of flight that was hers was more pronounced at the moment.
Suddenly there was a sound, and, as Mary thought, the book-shelves opened.

She saw light through the opening.